In organic chemistry, helicenes are ortho-condensed polycyclic Aromaticity in which Benzene or other aromatics are angularly annulation to give helix-shaped chiral molecules.
Structure and properties
The systematic naming for this class of compounds is based on the number of rings:
nhelicene is the structure consisting of
n rings. According to
IUPAC, only structures where
n is at least 5 are considered helicenes.
[ Some specific compounds also have alternate or . As the number of rings increases, starting at four, the structure becomes non-planar, but instead the planes of consecutive rings tilt to prevent steric collisions. For helicenes with six benzene units, a 360° turn is completed. In the helicene series the between the extremities increases going from 4helicene (26°) to 6helicene (58°) and then decreases again for example in 7helicene (30°).
]
Helicenes are notable for having chirality despite lacking both asymmetric carbons and . Instead, there is axial chirality, which results from the handedness of the helicity itself. The clockwise and counterclockwise helices are non-superposable. By convention a left-handed helix is minus and labeled ( M), a right-handed helix is plus and labeled ( P). Evidence from CD spectroscopy suggests left-handed helices are levorotatory and right-handed helices are dextrorotatory.
The stability of the two complementary helical enantiomers with respect to interconversion and the mechanism by which they interconvert depend on n.
Synthesis
The first helicene structure was reported by Jakob Meisenheimer in 1903 as the reduction product of 2-nitronaphthalene.
In one study,
Other approach is also non-photochemical and is based on assembly of biphenylyl-naphthalenes and their platinum-catalyzed double cycloisomerization leading to various 6helicenes:
[File:Tetrahelicene.jpg|[4Helicene]]
[File:Pentahelicene.jpg|[5Helicene]]
[File:Hexahelicene.jpg|[6Helicene]]
[File:Hexahelicene2.jpg|[6Helicene, other
chirality]]
[File:Heptahelicene.jpg|[7Helicene]]
[File:Heptahelicene2.jpg|[7Helicene, other chirality]]
[File:Octahelicene.jpg|[8Helicene]]
[File:Nonahelicene.jpg|[9Helicene]]
[File:Decahelicene.jpg|[10Helicene]]
[File:Undecahelicene.jpg|[11Helicene]]
[File:Dodecahelicene.jpg|[12Helicene]]
[File:Tridecahelicene.jpg|[13Helicene]]
[File:Tetradecahelicene.jpg|[14Helicene]]
[File:Pentadecahelicene.jpg|[15Helicene]]
[File:Hexadecahelicene.jpg|[16Helicene]]
[File:Octadecahelicene.jpg|[18Helicene]]
Applications
Helicenes have been studied with respect to nonlinear optics,[ conformational analysis,][ and hetero-atom substitution.]
See also
-
Other configurations of consecutively-fused benzene rings:
-
, linear
-
, zig-zag
-
, closed ring
- General references
